Mailinglist Archive: opensuse (3572 mails)

[James Knott <james.knott@xxxxxxxxxx>]

Aaron Kulkis wrote:
> James Knott wrote:
> > Doug McGarrett wrote:
> > > On Friday 15 February 2008 08:15, Carlos E. R. wrote:
> > > > The Thursday 2008-02-14 at 12:06 -0500, Aaron Kulkis wrote:
> > > > > >  I'm using one of these:
> > > > > >
> > > > > >
> > > > > > http://www.order.conrad.com/xl/1000_1999/1200/1250/1253/125319_AB_05_FB. 
> > > > > >
> > > > > > EPS.jpg
> > > > > I would look for a device with a needle rather than
> > > > > a digital readout.
> > > > I don't think you can measure true rms with a coil and needle. :-?
> > > >
> > > > It was measured by thermal effects...
> > > >
> > > > --
> > > > Cheers,
> > > >         Carlos E. R.
> > > There are electronic true-RMS voltmeters with analog readouts.
> > > For example, the Balantine 323 has a nice big dial.  You would have
> > > to measure the voltage drop across a known value shunt resistor in
> > > series with the load, and calculate the power from voltage and
> > > current.
> >
> > Ummm...  He was talking about RMS, which implies voltage or current. 
> > There's no such thing as RMS power.
>
> RMS is short for Root-mean-squared
> i.e.     _______
>         / _ 2
>        V  X
>
> RMS can apply to any function vs. time (voltage, current,
> power, etc.)
It's been a while since I've done it, but if you try calculating RMS 
power, you soon find your calculations are meaningless.  The purpose of 
the RMS calculation is to determine the equivalent DC voltage or current 
that would produce the same amount of power.  So, when you square a 
voltage or current sample you are in effect making an instantaneous 
power calculation P=E^2/R or P=I^2R, then taking the mean power value 
over the cycle and then converting back to a voltage or current value.


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